1. Field of the Invention
The present invention relates to a light-emitting element driving apparatus, a control method of the same, an optical encoder, and a camera.
2. Description of the Related Art
In an application such as an optical encoder, a light-emitting element used as a light source can be driven by APC (Auto Power Control) so as to hold the emitted light amount constant. As an example of an apparatus capable of this APC operation, a light-emitting element driving apparatus that monitors the emitted light amount of the light-emitting element and controls the driving of the light-emitting element so that the emitted light amount approaches a target value is known.
Japanese Patent Laid-Open No. 2001-244556 has disclosed a technique pertaining to the APC operation of a laser diode driving circuit. More specifically, a monitor signal Vm as a monitoring result of the emitted light amount of a light-emitting element 107 is input to the input terminal of a comparator 101, and the APC operation is performed based on the result of comparison performed on the monitor signal Vm and a reference voltage Vref1 by the comparator 101. Also, an amplifier circuit 110 is connected to the input terminal of the comparator 101 via a switch 111, and a signal of the amplifier circuit 110 is output to the input terminal by turning on the switch 111 before the APC operation is started. This method shortens the time elapsed before the emitted light amount of the light-emitting element stabilizes. However, the performance of the amplifier circuit 110 determines the time elapsed until this stabilization.
To increase the speed of the APC operation by shortening the activation time of the light-emitting element driving apparatus, the potential of a node for controlling the driving of the light-emitting element must be controlled within a short time period. For example, when reactivating an optical encoder after it is paused, the light-emitting element must return to the emitted light amount in an emitting state as a target value within a short time period. A possible practical example is a case in which, in order to acquire a difference between a signal component obtained by the optical encoder and the offset component of the apparatus, the offset component is acquired by setting the light-emitting element having emitted light in a non-emitting state, and then the light-emitting element is returned to the previous emitting state.
According to Japanese Patent Laid-Open No. 2001-244556, however, a predetermined signal determined by the amplifier circuit 110 as an external circuit is input to the comparator 101, and no signal corresponding to the emitting state of the light-emitting element is generated. Therefore, for example, it is difficult to perform control corresponding to characteristics resulting from the production tolerance and unique to the apparatus. Accordingly, the emitted light amount of the light-emitting element cannot return to the emitted light amount in an emitting state immediately preceding a non-emitting state within a short time period.